On the properties of the Front End Pool in Lync Topology Builder there is a field to enter the external web servers FQDN. Once you've filled this, remember to publish the topology.
Is your client showing an external web services URL at all or no URL at all?
Can you confirm that your External Web Site exists in IIS on the Front End server?
Also, try re-running the "Install or Update Lync Server System" step in the Deployment Wizard.
Normally, if you configura the External Web service FQDN in Topology builder and publish it, you can see it in lync client. If you still can not see it in lync client, please check the replication services in lync servers.
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